Broadcasting systems employing a radiated unmodulated carrier wave as a heterodyningsignal



June 26, 1962 L. w. PARKER 3,041,450

BROADCASTING SYSTEMS EMPLOYING A RADIATED UNMODULATED CARRIER WAVE AS AHETERODYNING SIGNAL Original Filed Nov. 27, 1956 2 Sheets-Sheet 1 LEGEND Mom/LA TED fiMsM/TTEZ DIRECTIONAL 5050 MC. Continuous Wave A 5000Continuous IN V EN TOR. LOUIS W. PARKE R ATTORNEYS June 26, 1962 v w.PARKER BROADCASTING SYSTEMS EMPLOYING A RADIATED UNMODULATED CARRIERWAVE AS A HETERODYNING SIGNAL Original Filed Nov. 27, 1956 2Sheets-Sheet 2 4 FIG. 4. 33 347 T 55 2 35 FREQ. FREQ. 5000 MC. INDI TCORRECTOR MODULATED y IOMC BOMG v .32

I so 3 J\ o M BEATFREQ. t 4 F COM/EH5 INDICATOR J 39 BEAT FREQ. 4940 MC.INDICATOR CONVERTER UMODULATED v TRANSM. v ww FREQ. GORRECTOR INVENTOR.LOUIS W. PARK ER ATTORNEYS Unite States Patent 3,041,450 BROADCASTINGSYSTEMS EMPLOYING A RADI- ATED UNMUDULATED CARRIER WAVE AS A HETERODYNWGSIGNAL Louis W. Par rcr, 375 Fairfield Ave, Stamford, Conn. Continuationof application Ser. No. 626,569, Nov. 27, 1956. This application Apr.25, 1960, Ser. No. 24,618 20 Claims. (Cl. 325-51) This invention relatesto ultra high frequency systems and more particularly to systems forbroadcasting television (or other wide band modulated waves) on ultrahigh frequency bands. In this respect the present invention comprises acontinuation of my prior application No. 626,569, filed November 27,1956 for Ultra High Frequency Systems, now abandoned, which priorapplication was in turn a continuation-in-part of my prior abandonedapplication Serial No. 288,238, filed May 16, 1952, for Ultra HighFrequency Systems; and Serial No. 280,927, filed April 7, 1952, PatentNo. 2,831,105, granted April 15, 1958, for Television DistributingSystem.

At present, superheterodyne receivers are used in ultra high frequency(UHF) or super high frequency (SHF) systems of the radio frequencyspectrum, but these receivers have several major disadvantages. One ofthese disadvantages resides in the fact that the degree of frequencystability required in the local oscillator is far greater than can beachieved with a simple inexpensive continuously tuned ultra highfrequency oscillator. The superheterodyne receiver also requires radiofrequency amplifier stages for blocking current flow from the localoscillator to the antenna and also for eliminating image responses. Thisinvention has as its primary object the provision of an ultra highfrequency system which avoids the disadvantages that inhere in ultrahigh frequency superheterodyne receivers.

The invention utilizes the principle that a modulated UHF or SHFtransmitter can broadcast to very high frequency (VHF) receivers withthe aid of a UHF or SEE C.W. transmitter which differs in frequency fromthe modulated transmitter 'by a frequency within the tuning range of theVHF receivers. These receivers employ a coverter at the antenna andchange the two UHF signals to a VHF signal.

An object of the present invention is to provide a system in which thismode of operation can be extended so that low-power UHF transmitters canserve VHF receivers in a large area. The system is extended to providecoverage to any given region by providing additional local C.W.transmitters. Such local C.W. transmitters are not synchronized withother C.W. transmitters covering adjoining regions, yet provision ismade for high quality performance even in the areas where two or more ofthe regions overlap. To this end the invention employs the phenomenonthat when several unmodulated frequencies of differing amplitude andfrequency operate a frequency converter by heterodyning with a modulatedfrequency, satisfactory operation can be achieved when one of theunmodulated frequencies has a peak amplitude at least as high as thearithmetic sum of the peak amplitudes of all the others.

To ensure the required predominance of one of the UHF C.W. signals, theinvention makes use of directional receiving antennas. This affords theopportunity for discrimination by directional pick-up at the receiver toprovide a predominant UHF C.W. signal along with the modulated UHFsignal. Special regional situations, such as shaded areas, are met byaddition of new modulated UHF transmitters at frequencies on theopposite side of the UHF C.W., separated by the same VHF, or by additionof new sets of modulated and C.W. UHF transmitters dfidlfifl PatentedJune 26, 1962 at frequencies other than those of the original sets andwith the same VHF separation.

Another object of this invention is to provide an ultra high frequencysystem in which the receivers are lower in cost than those at presentemployed.

Yet another object of the invention is to provide an ultra highfrequency system in which the necessity of crystal controlledoscillators, for the transmitter radiating the carrier, is avoided.

Another object of the invention is to provide an ultra high frequencyradio system in which a number of transmitters may be employed torespectively cover different areas not covered by the main transmitter,and in which there is no interference between the several transmittersthus employed.

Other objects and advantages of the invention will appear as thisdescription proceeds.

While I am describing my invention in detail in the followingspecification, it is understood that the broader aspects of thisinvention are not limited to the details herein disclosed. The scope ofmy invention is therefore being defined in the claims.

In carrying out the foregoing objects, I employ an ultra high frequencytelevision transmitter of conventional design operating at a givenfrequency of say 5,060 megacycles. Instead of a local oscillator at thereceiver, I transmit a continuous Wave heterodyning signal from anothertransmitter operating on say 5,000 megacycles. At the receiver, thesignals from the two transmitters are mixed in a special mixer which isa cavity resonator that resonates at the two frequencies of the twodifferent transmitters, along two different dimensions of the resonatorrespectively. There is a pick-up coil which constitutes an output forthe resonator and which has a signal therein which is a combination ofthe signals from the two transmitters. The mixed signals are fed througha crystal rectifier to produce a signal whose frequency is equal to thedifference in the frequencies of said transmitters. This latter signalis fed to a conventional very high frequency receiver and amplified inthe usual way.

The second transmitter has ten to one hundred times the power of themodulated one since the crystal rectifier requires a certain minimuminput in order to operate efficiently. To extend the range of the systemit is not necessary to increase the power of the modulated transmitterbut merely to place additional high power contlnuous wave transmittersthroughout the area into which the range is to be extended. Theseadditional transmitters should operate on or near the frequency of thefirstnamed continuous wave transmitter (5,000 megacycles 1n the examplegiven) although the frequency need not be exact.

Beyond the range of the amplitude modulated video signal I may employ anadditional video transmitter using another frequency, for example 4,490megacycles, while still using the same frequency (5,000 megacycles) forthe continuous wave signal. If there are a number of shaded areas sothat a number of amplitude modulated video signals are necessary, I canuse a duplicate system in addition to the first one, with thefrequencies of transmission of the duplicate transmitters say 50megacycles higher than the complementary signals of the first system.

Instead of using separate transmitters for the video signal and thecontinuous wave signal, I may employ a transmitter operating at say5,000 megacycles modulated at 60 megacycles with a low percentage ofmodulation. The video modulation is impressed on the 60 megacycle wavewhich is a subcarrier.

The frequencies of the transmitters remote from the main central one maybe controlled by directly or in- I than that of the video transmitter.

ferent transmissions, in the spectrum, according to one form of theinvention.

FIGURE 4 is a block diagram of apparatus capable of carrying out oneform of the invention.

In FIGURE 1, a video transmitter having the conventional amplitudemodulations on its carrier has an omnidirectional antenna 11. In closeproximity to it, although the distance is not critical, there is asecond transmitter emitting an unmodulated (continuous wave) signal fromits omnidirectional antenna 12. For reasons that will later appear, itis desirable for the signal from antenna 12 to be much stronger thanthat from antenna 11 and hence the continuous wavetransmitter has apower out put on the order of ten to one hundred times greater Whileother frequencies may be employed in the present illustration, thecarrier frequency of the modulated waves from antenna 111 is 5,060megacycles and that of the unmodulated waves from antenna 12 is 5,000megacycles.

FIGURE 2 illustrates a mixer used at each television receiver. Dipoleantenna 21 picks up signals from both antennas 11 and 12 and feeds thesesignals to loop 23 which excites the cavity resonator 22. The dimensionsH and V of this resonator 22 are slightly different from each other andare so selected that the resonator 22 will oscillate in one plane at5,000 megacycles and in the other plane at 5,060 megacycles. Exacttuning in the horizontal and vertical planes may be achieved by the useof adjusting screws 24 and 25' which cooperate with complementarymetallic capacitor elements 24a and 25a.

Both the exciting loop 23 and the pick-up loop 29 are mounted in a planewhich is displaced 45 degrees from the horizontal in order that theseloops may interchange energy with both of the oscillations existing inthe cavity resonator 22. Loop 29 is grounded at its lower end 28 to themetal resonator 22 while the upper end connects to the rectifier 26. Theoutput of the loop is fed to crystal rectifier 26 and thence to aconventional VHF receiver 27 which operates at 60 megacycles.

In order for the crystal rectifier 26 to operate Well as a mixer in mysystem it must receive a continuous wave signal (leaving antenna 12), ofat least a predetermined minimum value. Circle 13 of FIGURE 1illustrates the limit beyond which that potential cannot be obtained atthe crystal rectifier. The range is to a large degree detcrmined by thepower of the unmodulated signal radiated from antenna 12 for the reasonthat the crystal rectifier requires a certain minimum potential in orderto effectively mix the signals. Therefore, the unmodulated transmitterfeeding antenna 12 should have a power output many times that of themodulated transmitter, for example ten to one-hundred times. Since thereis a certain minimum power necessary for proper operation of crystalrectifier 26, the overall cost of the equipment may be reduced if thelarger power is transmitted as an unmodulated signal. Moreover, it ispossible to readily extend the range by merely adding additionalcontinuous wave transmitters at remote points, which is one of theimportant novel features of my invention.

Inasmuch as it is usually impractical to amplify the incoming signals atultra high frequencies, it is necessary that the crystal rectifier 26operate at some locations on very weak signals. The minimum value ofmodulated signal voltage can be very low since it only needs to be 4-above the noise level which may be in the order of 10 microvolts. Theminimum value of continuous wave signal voltage however must bedetermined by different considerations as is explained in the following:

The crystal rectifier 26 may operate in two different ways and the valueof the continuous wave signal voltage depends on which of these ways therectifier operates. If there is only one continuous wave signal present(with any number of modulated channels), there has to be a sufficientminimum voltage from the continuous wave to create appreciabledifference in the conductivity of the crystal for opposite directions ofcurrent. The lower limit for this voltage is in the order of onemillivolt with most good crystals. This type of crystal operation is infact known to the prior art.

I have discovered, however, that when there are a plurality ofasynchronous continuous wave signals present, differing from each otherby no more than about onehalf megacycle, my system will work perfectlywell with substantially no interference, as long as two additionalrequirements are satisfied at the crystal. First, that one of thecontinuous wave signals have a peak voltage amplitude which is higherthan the sum of all the peak voltages of the other continuous wave andmodulated signals present simultaneously at crystal rectifier 26. Thesecond requirement is that the crystal operate substantially on thestraight line portion of its voltage current characteristic curve. Thislatter requirement is satisfied by making the voltage of the largestcontinuous wave signal more than a certain minimum amount, depending onthe properties of the crystal. of voltage with presently availablecrystals is in the order of 0.1 volt peak. In addition, a low D.C.biasing voltage 44 may be added in series with the incoming signals toenable use of less voltage from this latter source. This D.C. biasingvoltage may vary from a few millivolts to 0.2 volt depending on theproperties of the crystal. However, the use of such bias is optional, asit is only necessary with moderately weak signals.

Without disturbing antennas 1 1 and 12 (together with their associatedtransmitters), I may extend the range by adding four antennas 14, 15,'16 and 17. All of these antennas are fed by continuous wavetransmitters operating at 5,000 megacycles (which frequency ismaintained with an accuracy defined by limits of plus or minus 200kilocycles). These transmitters are directional and respectively producelobes 14a, 15a, 15a and 17a. With this arrangement satisfactoryreception is possible Within any of the areas covered by lobes 13 and14a to 17a inclusive. For example, a receiver 18 will receive a powerfulcontinuous wave signal at 5,000 megacycles, from antenna 14 which issufficient to provide the necessary radio frequency potential of saidpredeterminedminimum value or more at the crystal rectifier 26. Whenthis condition has been met, the mixer of FIGURE 2 will operatesatisfactorily as long as the unmodulated signal from antenna 11produces the required potential at the crystal rectifier 26. It isapparent, therefore, that once a high power 'heterodyning signal isreceived, reception of the modulated signal is possible even though thereceiver is at a considerable distance from the antenna 11.

Unless the several continuous wave transmitters feeding antennas 12, 14,15, 16 and 17 are synchronized, it is desirable that they have a minimumof overlapping of their lobes. The arrangement shown in FIGURE 1 issatisfactory in this regard. If the frequencies of the continuous wavetransmitters are allowed to vary as much as 200 kilocycles it will notmatter. As long as the signal at the receiver fromone of the continuouswave transmitters is very strong it will not matter if weak continuousasynchronous signals on slightly different frequencies are alsoreceived. The signal received from the closest continuous wavetransmitter should preferably be three to four times as many volts asthat received from any other continuous wave transmitter. This conditionis not met at This minimum value a receiver 19 which is almost equallydistant from antennas 14 and 15; hence it would be desirable for thereceiver 19 to employ a directional antenna beamed at one or the otherof antennas 14 and 15, unless of course the transmitters feedingantennas 14 and 15 have their frequencies synchronized.

The reason why a radio frequency signal from antenna 15 will notinterfere appreciably with reception at receiver 18, assuming that thesignal from antenna 14 is several times stronger than that from antenna15 will now be explained. The signal from antenna 15 will have an effecton that from antenna 14- very much the same as a single sideband would.In other words it will amplitude and phase modulate the signal fromantenna 14. The theoretical explanation of this is elaborated upon in myUS. Patent 2,448,908 where 1 pointed out that a small amplitudemodulation, on a large amplitude carrier which carrier heterodynesanother carrier of small amplitude, will not be transferred to the beatfrequency. However, any modulation of the carrier of smaller amplitudewill be transferred to the beat frequency. In view of this, anyamplitude variation in the signal from antenna 14 will not affect theoutput signal from receiver 18. Any phase modulation caused by thesignal from antenna 15 will affect the more powerful signal from antenna14 and this phase modulation will pass through the entire receiver up tothe second detector. This detector is a simple rectifier and istherefore insensitive to phase modulation; consequently the output willbe the same as though there had been no phase modulation.

If reception in additional territory is desired an additional continuouswave transmitter feeding antenna may be employed in order to furnish alobe 20a covering the added territory.

It is understood that since the transmitters feeding antennas 14, 15, 16and 17 need not be accurately controlled that they may be self-excitedoscillators in which tuned circuits, as distinguished from crystals, arerelied upon to determine the frequency of transmission.

In cases where the continuous wave signal comes from a differentdirection than the signal from transmitter 11, two separately orienteddirectional antennas feeding a common cavity resonator may be used.Alternatively, each of the two directional antennas may feed separatecavity resonators whose outputs are combined. In the great majority ofcases the refinements mentioned in this paragraph are unnecessary.

Until now, only the continuous wave transmitters have been discussed inmultiple use and it has been assumed that the amplitude modulatedtransmitter, while having much reduced field strength at the distantlocations is still able to supply enough signal voltage to operate theaverage television receiver. This situation can be justifiably appliedto most medium or even large size cities. However, if hills or largebuildings create a shaded area, it is necessary to set up one or severalmore additional amplitucle modulated transmitters. In the case of thesetransmitters, the previously discussed idea of operating transmitters(such as feed antennas 14 to 17) on approximately the same frequencydoes not apply.

When and if it becomes necessary to use an additional modulatedtransmitter within the field of another, that new transmitter must use adiiferent frequency, unless the field strength of the oid transmitter iswell below 50 microvolts per meter through the entire region ofoperation. A very weak signal can be swamped by a much larger one butattention must be paid to the effect of the newly added transmitter onreceivers within the operating range of the older transmitter. In mostcases it will be found desirable to use another frequency for atransmitter operating in an area within close proximity of another.

Such new frequency however need not be noticeable to the operator of thereceiver. On line A of FIGURE 3 a situation is illustrateddiagrammatically where a set of modulated transmitters are located bothabove and below the frequency (5,000 megacycles) of the continuous wavetransmitter. Both higher (5,060 megacycles) and lower (4,940 megacycles)frequencies are spaced an equal frequency difference (60 megacycles)away from the unmodulated transmitter. Consequently, when either thehigher or lower frequencies are heterodyned with the continuous wavecarrier, the beat frequency is the same. This situation applies ofcourse not only to one but to any number of transmitters. Separation ofthe upper and lower frequency signals is accomplished by the cavityresonator at the receiving antenna and by the directionality of thepick-up system. The cavity resonator can differentiate about decibels,while the antenna array can differentiate up to 20 decibels. Hence, theunwanted signal may be reduced by 60 decibels, which is generallyconsidered satisfactory.

If the dead spots or shadows are not near one another, severaltransmitters on the same lower heterodyning frequency (4,940 megacyclesfor example) may be used at very low power (one for each dead spot), inaddition to the regular higher power transmitters on the upper frequencybands. Where this cannot be done without interference, another set offrequencies as shown on line B of FIGURE 3 will permit additionalseparation. Still the operator need not be aware that he is notreceiving the original transmitter, since the frequencies reaching hisVHF receiver will always be the same. The cavity resonators can be madeto have sufficient tuning range to cover all the frequencies that may beused. Tuning these units is accomplished by the Serviceman making theinstallation. After that, the operator of the receiver need not be awareof where the video signals originate.

The transmitters used in this system can be greatly simplified, duemainly to the abundant space allowable in the UHF range of the spectrum.

Instead of using a different transmitter and antenna I for thecontinuous wave signal than is used for the modulated signal, theantennas 11 and 12 as well as their transmitters may be combined asshown in FIGURE 4. Transmitter 35 may operate at 5,000 megacycles and itmay be modulated by the very high frequency video transmitter 30. If thepercentage of modulation exercised by the 60 megacycle transmitter 30 ontransmitter 35 is low, for example 30%, it is clear that the carrier oftransmitter 35 produces a signal corresponding to that leaving antenna12 of FIGURE 1. The 5,000 megacycle carrier has a 5,060 megacycle upperside band which is weaker than the carrier due to the low percentage ofmodulation. This upper side band corresponds to the signal radiated fromantenna 11 to FIGURE 1.

Transmitter 35 may also be modulated by video transmitter 31 operatingat 70 megacycles and also by video transmitter 32 operating atmegacycles. Therefore, as shown on line A of FIGURE 3 there will be acarrier at 5,000 megacycles and three upper side bands at 5,060, 5,070,and 5,080 megacycles. There will also be three lower side bands at4,940, 4,930 and 4,920 megacycles which may be eliminated by filters inevent that they would interfere with other transmissions or in the eventthat these lower frequencies are used by transmitters operating inshaded areas as described hereinabove. In any event, the VHF receiver 27of FIGURE 2 may select the program of any one of transmitters 30, '31 or32 by suitable adjustment of its tuning circuit.

Each of the 5,060, 5,070 and 5,080 megacycle frequencies may beconsidered as subcarriers modulated by a television signal. Thesesubcarriers have side bands that are nearly 5 megacycles wide, and whichcan be retained without a vestigial side band filter. However, thereceiver need only respond to one side band, say from 60 to 65megacycles as is now convention-a1 on vestigial side band receivers.Accuracy of frequency is required so far as the transmitters 30, 31 and32 are concerned, and hence these may be crystal controlled. The soundmay be transmitted as a 4.5 megacycle frequency modulated side band,added originally to the video signal modulating transmitters 30,31 and32. Transmitter need not be accurately frequency controlled, since thesomewhat loaded cavity resonator 22 on the receiving end may have a Q inthe order of 100. This corresponds to a bandwidth three decibels down ofmegacycles. Therefore, an inaccuracy of 0.1% (or five megacycles) willnot be noticeable. Crystal control is not necessary to get thisaccuracy, and therefore the numerous multiplier stages usually requiredmay be dispensed with. The frequency of transmitter 35 may be measuredin the well known manner by a cavity resonator 33. If the frequency hasdeviated from the assigned frequency by more than a few megacycles itmay be reset manually or automatically by the frequency corrector 34.

In FIGURE 4, I have shown tunable cavity resonators T (schematicallyillustrated as an inductor and a capacitor in parallel) for controllingthe frequencies of transmitters 35 and 36. These transmitters may beself-excited oscillators if desired. 7 V

With the apparatus described in FIGURE 4, a separate continuous wavetransmitter at the center of circle 13 is eliminated, but. simplecontinuous wave transmitters operating at or near 5,000 megacycles arestill employed to feed antennas 14, 15, 16, 17 and 29 in order toproduce the necessary signal strength at locations outside of circle 13.

The transmitters which feed antennas 14, 15, 16, 17 and 20 of FIGURE 1may conform to transmitter 36 of FIGURE 4. Crystal converter 37 picks upthe complete signal from transmitter 35. and also the 5,000 megacyclesignal from transmitter 36. In view of the very large amplitude of the5,000 megacycle signal from transmitter 36 at that location as comparedwith the amplitude of 5,000 megacycle signal from transmitter 35, the 60mega cycle beat note will be the result of the signal of transmitter 36beating with the upper side band (5,060 megacycles) of transmitter 35.The frequency of this beat note is measured by the frequency indicator33, and if incorrect may be reset manually or automatically by thefrequency corrector 39. Preferably this frequency should be held within0.12 megacycle of its assigned value.

As has been stated for additional coverage, the lower side bandfrequencies are sometimes eliminated by filters from the output oftransmitter 35, and these frequencies used by separate transmitterslocated in areas where inexpensive broadcast receivers cannot receivesignals from transmitter 35. Transmitter 4%) is such a transmitter andmay be used at location 50 of FIGURE 1 which is at the top of a hill 51that shades town 52 from direct reception of signals from antenna 11. Inthis case the converter 41 receives signals from both the 4,940megacycle transmitter 40 and the 5,000 megacycle transmitter 36 (whichmay be located at point 2% in FIGURE 1). The frequency corrector 43 isthen adjusted manually or automatically until the beat frequencyindicator 42 .indicates the desired frequency megacycles).

By the use of the invention as described hereinabove all the importantdisadvantages of UHF television broadcasting are eliminated. Theimportant disadvantage of oscillator radiation and too high arequirement for local oscillator accuracy at the receiver is solved byentirely eliminating the UHF local oscillator from the receiver.

Image rejection trouble is eliminated by using a fixed UHF frequency anda selective fixed tuned cavity resonator at the antenna. The cost of thereceiver is kept practically the same as that of a low frequencyreceiver. inating the need for very high frequency accuracy and by theuse of a simple automatic frequency control. Shadows in the transmittingrange are eliminated by using several small transmitters on differentfrequencies to boost the signal in such areas. These small transmittersdo not need operating personnel and may the turned on and oif by remotecontrol.

The cost of the transmitter is reduced by elim- 3. As has been statedhereinabove, the continuous wave signals from antenna 12 may have somemodulation on them without affecting the operation of the system,although preferably they should be pure continuous waves. Therefore, thewords continuous Waves are used in the claims to include not only purecontinuous waves but those which have so low a percentage of modulationthat they act in this system as continuous waves would act.

I claim to have invented: 1. A system for broadcasting ultra highfrequency modulated signals comprising means for producing a continuouswave signal and a modulated signal respectively on first and secondspaced ultra high frequencies, first antenna means for broadcasting saidsignals from a predetermined location, additional means for producing anadditional continuous wave signal at substantially said first frequency,and directional antenna means located near the limit of the effectiverange of the continuous wave signal for radiating the additionalcontinuous wave signal in a direction away from the first antenna means.2. The system of claim 1 in which the first antenna means isomni-directional, the continuous wave signals having relatively highpower as compared with that of the modulated signal.

3. The system of claim 2 in which there are a plurality of transmittersproducing continuous waves on substantially said first frequency, and aplurality of directional antennas fed by said transmitters forbroadcasting their outputs away from the first antenna means along radiioriginating at the first antenna means, the plurality of directionalantennas being spaced from each other and located near the limit of theeifective range of the signals from the first antenna means.

4. A system as defined by claim 1 having transmitter means forbroadcasting modulated signals on a third frequency, said second andthird frequencies being substantially equally spaced from the first oneand on opposite sides of the first one.

5. A system as defined by claim 4 in which the transmitter means islocated in an area outside of the elfective range of the first modulatedsignal, and means for broadcasting a continuous wave signal on the firstfrequency throughout the area covered by said transmitter means.

6. An ultra high frequency television broadcasting system comprisingmeans for generating an ultra high frequency video signal and afrequency modulated sound signal located adjacent to and outside theband of the video signal; means for generating an ultra high frequencycontinuous wave signal spaced from the video signal by a very highfrequency that falls in another band in which television stationsoperate; said continuous wave signal having relatively high powercompared to the video signal; omni-directional antenna means forradiating the signals generated by the first and second named means sothat the two may be heterodyned and rectified and then by means of avery high frequency receiver designed to operate in said very highfrequency band demodulated, throughout a given area covered by saidradiated signals; and means for extending the area in which receptionmay occur comprising a plurality of additional means for radiatingcontinuous wave signals on frequencies substantially the same as that ofthe second named means and radiating the additional continuous wavesignals primarily outside of the first named area so that in areasadjacent to and outside of the first named areas there are high powercontinuous wave signals in addition to, the low power radiations of thefirst named means, the first named means being the only means forsupplying to said extended areas video signals at the frequency on whichit operates; each of said plurality of additional means including adirectional antenna beamed away from the third named means and spacedfrom the others to reduce the areas covered by two of said plurality ofadditional means.

7. An ultra high frequency television broadcasting system as defined inclaim 6 in which the radiations from said directional antennas producefield strength throughout said extended area which is relatively largeas compared to the field strength of the video signals.

8. An ultra high frequency television transmission system comprisingmeans for generating an ultra high frequency video signal and afrequency modulated sound signal located adjacent to and outside thehand of the video signal; means for generating an ultra high frequencycontinuous wave signal spaced from the video signal by a very highfrequency that falls in another band in which television stationsoperate; said continuous wave signal having relatively high powercompared to the video signal; omni-directional antenna means forradiating the signals generated by the first and second named means sothat the two may be heterodyned and rectified and then by means of avery high frequency receiver designed to operate in said very highfrequency band demodulated, throughout a given area covered by saidradiated signals; and means for extending the area in which receptionmay occur comprising a plurality of additional means for radiatingcontinuous wave signals on frequencies substantially the same as that ofthe second named means and radiating the additional continuous wavesignals primarily outside of the first named area so that in areasadjacent to and outside of the first named area there are high powercontinuous wave signals in addition to the low power radiations of thefirst named means, the first named means being the only means forsupplying to said extended areas video signals at the frequency on whichit operates; each of said plurality of additional means including adirectional antenna beamed away from the third named means and spacedfrom the others to reduce the areas covered by two of said plurality ofadditional means; and additional means for broadcasting televisionsignals into an area covered by at least one of the continuous wavesignals on a frequency spaced from that of the second named means by thesame amount that the first named means is spaced from the second namedmeans, the two video signals being respectively on opposite sides of thefrequency of the second named means.

9. A system as defined in claim including in addition: additionaltransmitter means for broadcasting continuous wave and modulated signalsover an area not covered by the other transmitters, the last namedcontinuous wave and modulated signals being spaced apart the same as theother continuous wave and modulated signals whereby each set ofcontinuous wave and modulated signals have the same beat frequency, oneof the signals of the additional transmitter means having a frequencybetween said first and second frequencies, all of said modulated signalscarrying the same program.

10. In an ultra high frequency television broadcasting system as definedin claim 6; said first named means comprising a very high frequencysignal generator that modulates the second named means at said very highfrequency.

11. In combination, a system for broadcasting ultra high frequencymodulated signals as defined in claim 1, and receiving means locatedWithin the range of said system comprising resonant means fed by atleast one of the continuous wave signals and by the modulated signal andproducing an output which is at the beat frequency between those twosignals, rectifier means fed by the output of the resonant means andcomprising a rectifying element adapted to suppress the production ofbeats caused by the continuous wave signal from one of said antennaswhile passing beats caused by the other if the continuous wave signalshave widely different amplitudes, and a very high frequency receiver fedby the output of the rectifier means.

12. The combination recited in claim 11 including means, comprising adirectional antenna, for feeding the 10 resonant means to increase thedifference in amplitudes between the two continuous wave signals.

13. A system for broadcasting ultra high frequency signals comprisingmeans for transmitting a continuous wave Signal at a first ultra highfrequency, means for simultaneously transmitting modulated ultra highfrequency signals adapted to be heterodyned with said continuous wavesignal thereby to produce a very high frequency signal comprising a beatfrequency between said continuous wave and modulated ultra highfrequency signals, said last-named means comprising means transmittingtwo distinct modulated ultra high frequency signals from two differentlocations at second and third different ultra high frequenciesrespectively, said second and third modulated ultra high frequencysignals covering different areas of reception respectively, said secondand third ultra high frequencies of said two modulated signals beingsubstantially equally spaced from and on opposite sides of said firstfrequency of said continuous wave sign-a1, each of said second and thirdmodulated ultra high frequencies being spaced from said first continuouswave frequency by substantially the same very high frequency differencewhereby similar very high frequency beats are obtainable both above andbelow the frequency of said continuous wave signal upon the heterodyningof said continuous wave and modulated ultra high frequency signals.

14. In a system for broadcasting ultra high frequency modulated signalsas claimed in claim 6, a plurality of receiving means located Within thearea covered by said system each comprising resonant means responsive toboth said continuous wave and said modulated ultra high frequencysignals, crystal rectifier means fed by the 'output of said resonantmeans, amplitude selective means to cause one of the continuous wavesignals of comparable field strength received by said receiving meansfrom more than one of said omnidirectional and said additional antennameans to have a strength higher than the sum of the combined strengthsof the other continuous wave and modulated Signals simultaneouslypresent at the input of said receiving means for producing very highfrequency beat signals by said first continuous wave signal andsuppressing beat signals by the remaining of said continuous wavesignals, and a very high frequency receiver fed by the output of saidrectifier means.

15. A broadcasting system as claimed in claim 14, wherein said amplitudeselective means comprises directional receiving antenna means forfeeding said resonant means, to increase the difference in amplitudebetween different continuous wave signals present at the input of saidreceiving means.

16. In a radio broadcasting system, means for simultaneouslybroadcasting at least one modulated high frequency signal at a firstfrequency and a plurality of continuous wave high frequency heterodyningsignals all having a frequency approximately equal to a predeterminedsecond frequency, said first and second frequencies being located withinthe ultra high or super high frequency broadcast band and being spacedfrom one another by a frequency difference equal to a frequency in thevery high frequency broadcast band, each of said high frequencycontinuous wave signals being broadcast from different geographicallocations with a transmission power related to the transmission power ofsaid modulated high frequency signal to cause the received heterodyningsignals within the area covered by said system to be large compared withthe received modulated signal, receiving means located within said area,said receiving means including resonant means responsive to both themodulated and continuous wave ultra or super high frequency signals, andfurther including crystal rectifier frequency changing means connectedto the output of said resonant means for converting the receivedmodulated ultra or super high frequency signals into modulated signalsin the very high frequency broadcast band, means operatively associatedwith the receiving means located within regions of said area normallyreceiving signals of comparable field'strength from at least twocontinuous wave transmissions to cause one of the received continuouswave heterodyning signals to have a peakramplitude higher than thearithmetically combined amplitudes of the other heterodyning' and themodulated signal simultaneously present at the input of said receivingmeans, and said receiving means including a very high frequency receiverfed by the output of said rectifier means.

17. In a radio broadcasting system, transmitter means for simultaneouslybroadcasting at least one modulated high frequency signal having a firstcarrier frequency and a plurality of continuous wave high frequencyheterodyning. signals all having a frequency approximately equal to apredetermined second frequency, said first and second frequencies beinglocatedvwithin a first relatively high broadcast frequency band andbeing spaced from one another by a frequency diiference corresponding toa third frequency located within a second relatively lower broadcastfrequency band, each of said continuous wave high frequency signalsbeing broadcast from different geographical locations with atransmission power related to the transmission power of the modulatedhigh frequency signals to cause the received heterodyning signals withinthe area covered by said system to be large compared with the receivedmodulated signal, receiving means Within said area each includingresonant means responsive to both said modulated and said heterodyningfrequency signals, and further including unidirectional conductive meansconnected'to the output of said resonant means for converting thereceived modulated signals into a modulated signal within said secondfrequency band, amplitude selective means associated with the receivingmeans located within regions of said area normally receiving signalsof'said second frequency of comparable field strength from at least twoheterodyning frequency transmissions, to cause one of the heterodyningsignals to have a peak amplitude higher than the arith- 12 meticallycombined amplitudes of the other heterodyning and the modulated signalsimultaneously present at the input of said receiving .m'eanaand saidreceiving means including a receiver designed" for said lower frequencyband fed by the output of said unidirectional conductive means.

18. In a radio broadcasting system as claimedin claim 17 includingbeamed transmission means for at least part of said heterodyningsignals, to reduce the size of said regions within said area.

1. In a radio broadcasting system as claimed in claim 17, wherein saidamplitude selective means is comprised of directional receiving antennameans to cause one of the received heterodyning signals originating fromone of the continuous wave transmissions to dominate the signalsoriginating from the: other continuous Wave transmissrons.

20. In a broadcast system as claimed in claim 17, means for broadcastingsaid modulated high frequency signal and one of said continuous wavehigh frequency signals being comprised of a high frequency generatorproducing a signal at said first frequency, means to produce a modulatedcarrier signal at said third frequency, and further means to amplitudemodulate said first signal by said last-mentioned modulated signal.

References Cited in the file of this patent UNITED STATES PATENTS2,140,730 Batchelor Dec. 20, 1938 2,425,352 Sloss Aug. 12, 1947 FOREIGNPATENTS 596,053 Germany Apr. 26, 1934 OTHER REFERENCES Microwave Mixers,vol. 16, MLLT. Series, McGraw- Hill, 1948, sec. 2', 4 pp. 56-59.

